Modem switching circuits use power transistors as switching elements. In a typical switching circuit, one or more power transistors are mounted, together with other components, on a printed circuit board. Two buses connected to a power supply provide the power that the transistors are to switch on and off. The printed circuit board, together with all the components mounted on the board, form a switching module, or transistor module. A switching circuit typically includes many of these transistor modules.
A difficulty associated with the design of high-power switching circuits is that the transistor modules within the switching circuit are often afflicted with parasitic inductance. This parasitic inductance is caused, in part, by currents flowing through the power buses that feed the various components of the transistor module. Inductance in an electronic circuit is analogous to mass, or inertia, in a mechanical system. Just as an ocean liner cannot quickly stop and change direction because of its mass, a switching circuit with high inductance cannot switch current on and off quickly. In addition, just as an ocean liner cannot easily be stopped at a precise location, a switching circuit with high inductance cannot easily switch from one level to another without briefly overshooting the target level.
The tendency of a switching circuit to overshoot its desired target level can cause considerable difficulties when, as is the case with power inverters, the transistor modules within the switching circuit handle large amounts of power. The voltage overshoot, which is caused in part by the parasitic inductance of the transistor modules, can reach hundreds of volts. This is enough to cause a failure within a transistor module and the destruction of its constituent components.
The present invention reduces the effective inductance of a switching module in a switching circuit by providing at least one additional conducting surface for current flowing on the power bus. The power bus and the additional conducting surface, when electrically connected, define a distributed bus having an inductance that is less than the inductance of the bus acting alone.
The additional conducting surface, or conducting sheet, can be on a second printed circuit board separate from the first printed circuit board on which the power bus is disposed. Alternatively, the conducting sheet can be on another surface or in the interior of the same printed circuit board on which the power bus is disposed.
When the bus and the conducting sheet are on separate printed circuit boards, conducting posts extend between the two printed circuit boards to support one printed circuit board opposite the other and to provide electrical communication between the power bus on one printed circuit board and a conducting sheet on the other printed circuit board.
When the power bus and the conducting sheet are on the same printed circuit board, a via provides electrical communication between the power bus and the conducting sheet. In the case of a multi-layer printed circuit board, the via can penetrate up to a selected depth into the printed circuit board to reach a conducting sheet buried in an inside layer of the board. In the case of a single layer printed circuit board in which the conducting sheet is on the opposite side of the board from the power bus, the via can penetrate the printed circuit board to provide electrical communication between the power bus and the conducting sheet.
Regardless of whether the conducting sheet is on the same printed circuit board or a different printed circuit board, the use of a conductive post or a via enables the switching module having a distributed bus to maintain the same footprint as a conventional switching module.
The switching module of the invention typically includes a first printed circuit board for mounting a switching element. The first printed circuit board has a first power bus, for connection to a positive terminal of an external power supply for providing power to a switching element mounted on the first printed circuit board. A first conducting sheet is electrically connected to the first power bus. Together, the first conducting sheet and the first power bus form a distributed bus having a distributed bus inductance that is less than the inductance of the power bus. The first conducting sheet can be disposed on the first printed circuit board or on a second printed circuit board.
In the case in which the first conducting sheet is on a second printed circuit board, one or more conducting posts extending between the first and second printed circuit boards provide electrical communication between the first power bus and the first conducting sheet. In the case in which the first conducting sheet is on the first printed circuit board, one or more vias penetrating into the board can provide electrical communication between the first power bus and the first conducting sheet.
The first printed circuit board can also has a second power bus connected to a negative terminal of the external power supply and providing power to the switching element. The first and second power buses are electrically connected to first and second conducting sheets. These conducting sheets can be side-by-side on a common surface of either the first printed circuit board or a second printed circuit board, or on opposed surfaces of a second printed circuit board. Alternatively, the conducting sheets can be buried in the interior of a multi-layer board.
Conducting posts extend between the first and second printed circuit boards to provide electrical communication between the first power bus and the first conducting sheet and between the second power bus and the second conducting sheet. If necessary, a via can extend into or through a printed circuit board to provide electrical communication to a conducting sheet that is not easily accessible with a conducting post.
To further reduce the effective inductance of the transistor module, the first and second conducting sheets can be connected by a capacitance. The presence of a capacitance further reduces the voltage overshoot that accompanies rapid switching by the transistor module.
By reducing the effective inductance of the transistor module, the distributed bus of the invention enables the circuit designer to reduce the rise and fall times (collectively referred to as the xe2x80x9ctransition timesxe2x80x9d) of the transistor module""s output square wave. The reduction in transition time results in lower switching losses and a reduction in voltage overshoot. As a result, the distributed bus enables more efficient operation without a significantly increased risk of excessively high transient voltages. By providing a conducting sheet on the first printed circuit board or on a second printed circuit board opposed to the first printed circuit board, the transistor module with reduced inductance maintains the same footprint as a conventional transistor module.
These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: